The present invention relates to a fuel injection timing control apparatus for a diesel engine, whereby the timing of fuel injection into the cylinders of the engine is controlled on the basis of injection time phase difference deviations and firing time deviations.
There have been various proposals in the prior art for a fuel injection timing control apparatus having similar objectives to those of the present invention. For example Japanese patent No. 58-70029 describes an apparatus in which an actual firing time sensor is employed to detect the points in time at which firing (i.e. combustion of fuel being injected into a cylinder) actually occurs. This sensing is performed by detecting light which is emitted by the firing, e.g by an opto-electric sensing element disposed outside a viewing window. A target firing time is established, and feedback control is applied by a fuel injection timing control means such as to vary the timing of fuel injection into the cylinders until the actual firing time is made coincident with the target firing time. However, such an apparatus presents problems due to the fact that under some engine operating conditions, for example during intervals in which the engine is run without fuel being injected into the cylinders, or when the engine is idling with only very small amounts of fuel being injected into the cylinders, or when the viewing window is obscured by soot, etc, satisfactory signals will not be produced from the firing time sensor.
In order to overcome this problem, it has been proposed, for example in Japanese patent No. 59-153942, to switch over to an open-loop control of the fuel injection timing when satisfactory signals cannot be obtained from the sensor means used to detect the actual firing times. However such open-loop control cannot provide precise control of the fuel injection timing. Another attempt to overcome this problem is proposed in Japanese patent No. 58-20935, in which changeover to control of the fuel injection timing on the basis of a rotational phase difference is performed, when the firing time sensor means becomes inoperative as described above. However such control cannot provide a sufficiently high degree of accuracy, due to errors which arise from the mechanical tolerances of the fuel injection pump and engine components, inaccuracies resulting from component wear over a long period of use, etc.
It can thus be understood that prior art types of apparatus for controlling the fuel injection timing of a diesel engine based on firing times do not provide a highly accurate control over the entire range of engine operating conditions.